A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. A lithographic apparatus can be used, for example, in the manufacturing of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
To increase production rate of scanned patterns, a patterning device is scanned at constant velocity across a projection lens, back and forth along a scan direction. Therefore, starting from rest, the reticle quickly accelerates to reach the scan velocity, and then at the end of the scan, it quickly decelerates to zero, reverses direction, and accelerates in the opposite direction to reach the scan velocity. The acceleration/deceleration rate is, for example, 15 times the acceleration of gravity. There is no inertial force on the reticle during the constant velocity portion of the scan. However, the large inertial force encountered during the acceleration and deceleration portions of the scan, for example, 60 Newtons (e.g., =0.4 kg of reticle mass×150 m/sec2 of acceleration) can lead to slippage of the reticle. Such slippage can result in a misaligned device pattern on a substrate.
Further, the reticle is constrained in the Z axis direction (i.e., the gravitational direction) by the use of multiple supports. The function of such supports is to position the reticle in the Z direction while not affecting movement in the X and Y directions. In some configurations, the Z supports entail the use of an adhesive to affix a clamp that holds the reticle in place with the remaining support structure, e.g., a chuck. However, the use of adhesive results in significant XY stiffness and large hysteresis issues. Such hysteresis occurs during the acceleration and deceleration portions of the scan when the Z supports, attached to a clamp with adhesive are subject to XY shearing forces where the adhesive, while stiff, exhibits some amount of flexibility and hysteresis in the XY directions. In addition, as these Z supports have a non-zero stiffness factor they also generate a high stress point at the interface of the reticle and clamp in the surrounding regions, thus causing microslip and corresponding overlay issues.